C++ Copy and swap


Example

If you're writing a class that manages resources, you need to implement all the special member functions (see Rule of Three/Five/Zero). The most direct approach to writing the copy constructor and assignment operator would be:

person(const person &other)
    : name(new char[std::strlen(other.name) + 1])
    , age(other.age)
{
    std::strcpy(name, other.name);
}

person& operator=(person const& rhs) {
    if (this != &other) {
        delete [] name;
        name = new char[std::strlen(other.name) + 1];
        std::strcpy(name, other.name);
        age = other.age;
    }

    return *this;
}

But this approach has some problems. It fails the strong exception guarantee - if new[] throws, we've already cleared the resources owned by this and cannot recover. We're duplicating a lot of the logic of copy construction in copy assignment. And we have to remember the self-assignment check, which usually just adds overhead to the copy operation, but is still critical.

To satisfy the strong exception guarantee and avoid code duplication (double so with the subsequent move assignment operator), we can use the copy-and-swap idiom:

class person {
    char* name;
    int age;
public:
    /* all the other functions ... */

    friend void swap(person& lhs, person& rhs) {
        using std::swap; // enable ADL

        swap(lhs.name, rhs.name);
        swap(lhs.age, rhs.age);
    }

    person& operator=(person rhs) {
        swap(*this, rhs);
        return *this;
    }
};

Why does this work? Consider what happens when we have

person p1 = ...;
person p2 = ...;
p1 = p2;

First, we copy-construct rhs from p2 (which we didn't have to duplicate here). If that operation throws, we don't do anything in operator= and p1 remains untouched. Next, we swap the members between *this and rhs, and then rhs goes out of scope. When operator=, that implicitly cleans the original resources of this (via the destructor, which we didn't have to duplicate). Self-assignment works too - it's less efficient with copy-and-swap (involves an extra allocation and deallocation), but if that's the unlikely scenario, we don't slow down the typical use case to account for it.

C++11

The above formulation works as-is already for move assignment.

p1 = std::move(p2);

Here, we move-construct rhs from p2, and all the rest is just as valid. If a class is movable but not copyable, there is no need to delete the copy-assignment, since this assignment operator will simply be ill-formed due to the deleted copy constructor.